You need the INJECTOR to begin the Write process. If you don't add Archive2.dat to your backup the PS3 writes the INJECTOR only. You need to create yourself the INJECTOR, in fact, it's unique for backups. If you have an old INJECTOR you can use it.

Here's a video demonstrating the process, with a guide below:

== HOW TO ==

1) Switch on your PS3
2) Go to Settings -> System Settings -> Backup
3) Connect a USB Drive or a Memory Stick to PS3
4) Select it in Backup utility
5) Make a Backup
6) Copy it to your Desktop
7) Delete backup from the USB Device or Memory Stick
8) Connect it to the PS3
9) Delete something in your PS3 (To make backup smaller)
10) Backup again and copy the 2nd backup to your desktop
11) Delete backup from the USB Device or Memory Stick

Now you have 2 different backups on your desktop.

1) Copy archive2.dat and archive2_00.dat from the bigger backup to the smaller one (see picture below).

4) Once restore is complete, your PS3 has the files from the two Backup in HDD.

That means the PS3 writes files in the HDD, even if the backup is mixed with another. That can be useful in further hacking when you need to write a file in a directory, for example the Game directory.